Arsenic (As) is a pervasive and ubiquitous environmental toxin that has created catastrophic human health problems world-wide.Chlamydomonas reinhardtii is a unicellular green alga,which exists ubiquitously in freshwater aquatic systems.Arsenic metabolism processes of this alga through arsenate reduction and sequent store and efflux were investigated.When supplied with 10 μmol/L arsenate,arsenic speciation analysis showed that arsenite concentration increased from 5.7 to 15.7 mg/kg dry weight during a 7-day period,accounting for 18％-24％ of the total As in alga.When treated with different levels of arsenate (10,20,30,40,50 μmol/L) for 7 days,the arsenite concentration increased with increasing external arsenate concentrations,the proportion of arsenite was up to 23％-28％of the total As in alga.In efllux experiments,both arsenate and arsenite could be found in the efflux solutions.Additionally,the efflux of arsenate was more than that of arsenite.Furthermore,two arsenate reductase genes of C.reinhardtii (CrACR2s) were cloned and expressed in Escherichia coli strain WC3 1 10 (AarsC) for the first time.The abilities of both CrACR2s genes to complement the arsenatesensitive strain were examined.CrACR2.1 restored arsenate resistance at 0.8 mmol/L.However,CrACR2.2 showed much less ability to complement.The gene products were demonstrated to reduce arsenate to arsenite in vivo.In agreement with the complementation results,CrACR2.1 showed higher reduction ability than CrACR2.2,when treated with 0.4 mmol/L arsenate for 16 hr incubation.